Traditional silicon-on-insulator (SOI) and fully depleted SOI (FDSOI) transistor devices typically have a gate defining a channel interposed between a source and a drain formed within an active region of an SOI or FDSOI substrate. Such SOI or FDSOI devices have several advantages over devices formed on conventional bulk substrates: the elimination of latch-up, improved radiation hardness, dynamic coupling, lower parasitic junction capacitance, and simplified device isolation and fabrication. Such advantages allow semiconductor device manufacturers to produce low-voltage, low-power, high-speed devices thereon. For example, metal-oxide semiconductor field effect transistors (MOSFETs) are commonly formed on FDSOI structures. However, MOSFETs formed on such FDSOI structures suffer from a short-channel effect.
The short-channel effect refers to the effect caused by reducing a “long” channel to a “short” channel. A channel length is “long” where a depletion layer formed under the channel has a depth equal to its length. When a channel length is shortened to an extreme, the depletion layer extending from the drain side spreads in the direction of the source region and contacts with the depletion layer of the source side.
As a result, a potential barrier in the vicinity of the source is lowered by the drain voltage and a current (Ioff) flows by itself even when no voltage is applied to the gate electrode. In this case, an energy band between the source and the drain changes continuously. This is the punch-through effect which lowers the withstand voltage between the source and the drain.
While various countermeasures have been taken to reduce the short-channel effect described above, a general measure, which has been taken most frequently, is channel-doping. Channel-doping is a technique for suppressing the short-channel effect by doping a trace amount of an impurity element such as P (phosphorus) or B (boron) shallowly in the channel region (as disclosed in Japanese Patent Laid Open Nos. Hei. 4-206971, 4-286339 and others).
However, the channel-doping technique has a drawback in that it significantly restricts the field effect mobility (hereinafter referred to simply as a mobility) of the MOSFET. That is, the movement of carriers is hampered by the intentionally doped impurity, thus dropping the mobility.
Therefore, there exists a need in the art for a device formed on a fully depleted semiconductor-on-insulator structure with increased performance and better characteristics enhanced by improving an off state (Ioff) while maintaining a high on state (Ion).